1. Field of the Invention
This invention relates to diverse fields, like the food industry, the cosmetic industry, the perfume and scent industry, the chemical industry, the packaging industry, and the environment, and, more generally, to all fields in which the detection of odors and odorous substances or volatile substances, referred to generally, and without distinction, in the rest of this specification as odorous substances or odors, contributes means of investigation, of monitoring and action in these fields.
2. Description of the Prior Art
The detection of odorous substances has many industrial applications, especially in processes in the foodstuff industry, in which one can, for example, determine the degree of freshness and the quality of the products, thanks to the odorous substances which they release. Gas chromatography, which consists in a method of selective detection by separating the molecules of gas compositions, is used as a way of monitoring odors. However, this method has many drawbacks; in effect, the detection results of the substances vary according to the method of sampling chosen, the time taken to achieve the results is relatively long, and the cost of gas chromatography equipment added to the cost of a mass spectrometer necessary to analyze the results is high.
Recent developments in the field of detection of odorous substances propose, as means of detecting odorous substances, detection methods and apparatus using semiconductive gas-sensors. The odorous substances change the electrical properties of the semiconductors by making their electrical resistances vary, and the measurement of these variations allows one to determine the concentration of odorous substances. These methods and apparatus used for detecting odorous substances use a relatively brief detection time, of around a few seconds, compared to those given by gas chromatography, which takes from several minutes to several hours. On the other hand, the semiconductive gas-sensors must operate at a high temperature, around 350.degree. C. Moreover, the sensors have good sensitivity but a mediocre selectivity with regard to odorous substances.
Other recent gas sensor technologies are now used, leading to other methods and apparatus for detecting odorous substances. One finds, for example, apparatus having conductive-polymer gas-sensors and apparatus having surface-acoustic-wave gas-sensors.
The conductive-polymer gas-sensors have a film made of a conductive polymer sensitive to the molecules of odorous substances. On contact with the molecules, the electric resistances of the sensors change and the measurement of the variation of this resistance enables the concentration of the odorous substances to be determined. An advantage of this type of sensor is that it functions at temperatures close to room temperature. One can also obtain, according to the chosen conductive polymer, different sensitivities for detecting different odorous substances. In general, conductive-polymer sensors have better selectivity but are less sensitive than the semiconductive sensors.
The surface-acoustic-wave gas-sensors generally include a substrate with piezoelectrical characteristics covered by a polymer coating which is able to absorb the odorous substances. The variation of the resulting mass leads to a variation of its resonant frequency. This type of sensor allows for very good mass-volume measures of the odorous substances.
The methods and apparatus for detecting odorous substances that one normally encounters generally include several sensors so as to augment the selectivity of the odorous substances, but all of the sensors use the same technology, that is to say either semiconductive or conductive-polymer or surface-acoustic-wave technology. Because of this, the possibility of detecting odorous substances is limited to the level of sensitivity and the discrimination given by the sensors of one technology. Moreover, these apparatus use methods of transporting the odorous substances by natural convection, in a detection enclosure comprising the sensors, which increases to an appreciable extent the time it takes to sense the odorous substances and, thus, the duration of the detection processes, and this to a greater degree when the volatility of the product being tested is weak.